1. Field of the Invention
The invention relates to a DC power supply arrangement for a telecommunication line, the arrangement being connectable for its energy supply to a supply source earthed at one terminal, the arrangement having between its output terminals an output capacitor one connection point of which is directly coupled to a terminal of the supply source, the output voltage of the arrangement being controllable to a value lower than the supply source voltage by supplying a controllable charge to such output capacitor under the control of a periodic control signal and having a specified control range.
2. Description of the Related Art
Such a supply arrangement is known from the published report of the "Conference on Communications Equipment and Systems", Apr. 20-22, 1982, organized by the Electronics Division of the U.K. Institution of Electrical Engineers, entitled: "A Semiconductor SLIC", pp. 44-48, more specifically FIG. 1.
The supply arrangement described in the aforementioned report comprises a Switched Mode Power Supply (SMPS) having an output capacitor connected to a telecommunication line. One side of this output capacitor is connected to the earthed positive terminal of a supply source, the other side of it is connected to the negative terminal of the supply source via a self-inductance and a switching element of the supply arrangement.
Switched Mode Power Supplies have the advantageous phenomenon of being capable of deriving a controllable DC current from a fixed supply voltage, the dissipation in these power supplies being negligibly small. The adjustment of the output voltage of these power supplies takes place by means of a control signal varying the duty cycle and/or the switching frequency of the switching element, as a result of which the charge supplied to the output capacitor is controlled.
As a rule two types of currents can generally occur on a telecommunication line: differential mode and common mode currents. When the telecommunication line is in operation under normal and trouble-free circumstances only differential mode currents will occur in this line; these currents are equally large and of opposite direction in the two wires of the line. However, as a result of external disturbances (lightning inductances, short-circuit or asymmetry of the energy network) common mode currents can also be produced in the telecommunication line. These currents have the same direction in the two line wires. These common mode currents have to be discharged to earth, possibly finding a path through the output capacitor. The output capacitor can consequently be charged or discharged. If the output capacitor is discharged this will be compensated for by, for example, an increased duty cycle of the switching element. If the output capacitor is charged, and if the charging current exceeds the discharging current of the load, the voltage across the output capacitor will rise more and more without compensation for this by variation of the duty cycle of the switching element being possible. The voltage across the output capacitor can even rise so high that it hinders the signal traffic or even makes signal traffic impossible. Under these circumstances the telecommunication line is useless for communication purposes. In addition, too high a voltage on the output capacitor can cause too high a dissipation in a (usually integrated) subscribers circuit which may be inserted between the supply arrangement and the telecommunication line.
It might be possible to reduce the excessive output voltage by inserting in parallel with the output capacitor a controllable dissipating element, for example a transistor. However, in modern telecommunication exchanges, having a high packing density of line connection circuits, dissipating elements are undesired.
It might also be possible to avoid an excessive output voltage caused by the common mode currents by selecting an output capacitor of a high value. However, a drawback of this possibility is the volume of such a component. In addition, a large output capacitor can form a low impedance for signal voltages on the telecommunication line, such a capacitor thus forming an undesired load for this line.